1. Field of the Invention
This invention is in the field of methods and apparatus for the separation of particles from the liquid in which they are carried. More particularly, the present invention is in the field of method and apparatus for removing particulate polymer from the stripping solutions used in the processing of a printed wireboard (PWB).
2. Description of the Prior Art
Apparatus of the prior art for processing a PWB comprises means for coating a supported copper sheet with a photopolymer, or photoresist, material known as "resist", exposing the coated sheet to radiation to produce a pattern, developing the pattern on the resist, removing the undeveloped portions, etching the exposed copper, and stripping the remaining resist portions by contacting the treated PWB to the action of a stripping medium, whereby a dispersion of resist in the stripping medium is produced.
The stripping solution is used to remove resist from a PWB in order to leave a circuit pattern of desired configuration; the stripping solution must thereafter either be discarded, or the resist removed from it in order to permit re-use of the solution. Economics would appear to dictate the latter, but in the present state of the art, simply filtering the liquid generally requires changing or otherwise dealing with the filter medium and apparatus employed, with attendant problems of plugging, maintenance and disposal. Further, filtration of the particulate-containing stripping solution to effect the desired separation has heretofore been both time-consuming and labor-intensive, and thus of borderline economy.
The method presently in general use for the separation of resist particles from stripping comprises the use of a hydrocyclone filter, in which the solution is pumped through a cone which causes the liquid to spin, thereby imparting a centrifugal force, which effects a separation, by filtration through a screen, of about 40% of the resist particles from the solution. The resist particles not removed by the filtration, together with the filtered solution, are recycled to the stripping operation. The retained particles are removed from the filtration screen by any convenient means, such as scraping or hydraulic flushing, followed by disposal.
The retention of the majority of the particles in the filtered solution causes a progressive diminution of the efficiency of the solution to act as a resist-stripping agent, and results in the necessity for relatively frequent replacement of the filtered solution with fresh material, on an approximately weekly or more-frequent basis. This replacement of stripping solution is generally accompanied by shutting down the PWB production line, and is thus undesirable in terms of lost production time and non-productive labor cost.
A number of other approaches to the problem of separating the stripped resist from the stripping solutions are found in the prior art. Wong, in U.S. Pat. No. 4,760,014, describes apparatus for production and stripping of PWB; he teaches the addition of an acid to the resist-stripper solution in order to coagulate the resist into substantially non-tacky discrete particles which can then be removed from the liquid phase with relative economy, and with proper disposal then made of the respective portions of the stream. It is noted that the treatment method of Wong requires disposition of the stripper solution due to its altered condition after the separation step.
In a more general solid-liquid separation device described in U.S. Pat. No. 4,661,253, Williams provides the separation of animal waste from an aqueous carrier stream by allowing the carrier stream to flow over a screen maintained at an angle of substantially 60.degree. to the horizontal, while a high-pressure, oscillating water spray impinges on the lower surface of the screen and prevents clogging of the screen holes with particulates.
In U.S. Pat. No. 4,626,329, Uphus provides physical separation of liquids and solids by causing a carrier stream having solids dispersed therein to flow over a screen capable of varying angular inclination to the horizontal. Uphus provides an inverted funnel and baffle means to decrease the rate of the carrier stream and to distribute its flow to the screen in a substantially laminar fashion. The apparatus has no means for recirculation of the separated liquid to the process wherein that liquid became associated with the solids from which the Uphus apparatus separates it.
Walker, in U.S. Pat. No. 3,833,123, discloses apparatus having an inclined screen to effect separation of a carrier stream into its solid and liquid components, and includes baffles to release the boundary flow of the filtrate from surface-tension forces and to cause the filtrate to fall away from the lower surface of the screen. In a fashion similar to the Williams device, Walker uses a spray to reduce the tendency of the screen to clog. None of the prior-art teachings disclose a mechanism for minimizing the dwell time of the process.